Method of protecting magnesium and high grade magnesium alloys from the action of corroding agents



Patented Nov. 13, 1934 UNITED STATES PATENT OFFICE METHOD OF PROTECTING MAGNESIUM AND HIGH GRADE MAGNESIUM ALLOYS FROM THE ACTION OF CORRODING AGENTS Delaware No Drawing. Application April 1, 1931, Serial 4 Claims.

The present invention relates to a method of protecting magnesium and high grade magnesium alloys from the action of corroding agents,

particularly water, alcohols, and non-oxidizable 5 saline fluxes, at ordinary and elevated temperatures.

It has long been known that the presence of alkali chromates or chromic acid in a medium which exercises a corroding influence upon magnesium and high percentage magnesium alloys per se, causes a passivation of the metal which results in a substantial reduction or even a complete cessation of the corroding efiect. It has likewise been known that a treatment of these metals with aqueous solutions containing chromic acid or alkali chromate causes the formation of a thin protective layer upon the surface of the metal or work-piece which increases its resistance to a subsequent corroding action quite generally. Simultaneously, a coloring effeet is produced upon the metal surface.

I have now found that aqueous solutions of alkali sulfides cause the formation of a coherent protective layer upon magnesium and high percentage magnesium alloys. I have further discovered that exceedingly small additions of alkali sulfides when made to water, alcohols, or nonoxidizing saline fluxes and similar corroding agents, all of which are referred to in-the appended claims as liquid medium, have the effect of efliciently counteracting the corroding influence of these substances. It is remarkable and of the utmost importance that the corrosive action, particularly of alcohols, is even counterbalanced at comparatively high temperatures.

Several modifications of the present invention will hereinafter be described in detail. However, -I do not wish to limit the scope of my invention to such modifications, as other equally advantageous modes of applying the invention will occur to persons skilled in the art.

(1) The heat treatment of magnesium and high grade magnesium alloys at temperatures below the melting point of the metal is frequently carried out in a bath of non-oxidizing saline fluxes. The corroding action of such fluxes, which is generally but small per se, is entirely suppressed by the presence of a small quantity of alkali sulfides (sulfide of potassium, sulfide of sodium, or liver of sulfur in amounts ranging up to about 3 percent by weight of the saline flux employed).

(2) It is generally recognized that ordinary water has a slow corroding efiect upon articles made of magnesium and high percentage magne- In Germany April 4, 1930 sium alloys even at room temperatures and that this efiect becomes considerably accelerated at elevated temperatures. According to the present invention an addition of about 0.2-0.5 percent of alkali sulfides is made to the water with the result that the corrosive action of the water is entirely suppressed. The small increase in weight of the articles which .is frequently observed is due to the formation of a thin protective layer on the surface of the articles.

In the case of water'from a water main system or from rivers, the addition of alkali sulfides causes a precipitation. It is advantageous to separate such precipitate from the water by filtering or decanting off before bringing the water into contact with magnesium or high percentage magnesium alloys, as the precipitate when settling on the surface of the metal may give rise to a pitting effect upon the latter.

(3) An increased resistance to corrosion of magnesium and high grade magnesium alloys when in contact with alcohols such as glycol, glycerine, etc., or aqueous mixtures of such alcohols, particularly at elevated temperatures, is of the utmost importance as the substances and mixtures mentioned are frequently employed as cooling liquids in internal combustion engines so as to enhance the cooling efiiciency and to reduce the danger of freezing. The figures given in the following table are representative of the increased resistance to corrosion of magnesium and high percentage magnesium alloys in contact with glycol, glycerine and mixtures oi both with water due to the presence of alkali sulfides. The figures given in the last column represent the corroding effect by the reduction in weight of the tested articles in grams per square meter of surface per 24'hours.

Loss in I weight in grams er Temperature and composition of the liquid sq per 24 hrs.

Glycol 0 1c. 6 Glycol +0.1 percentum sulfide of potassium 0 Glycol 150 C 56. 5 Glycol +0.3 percentum sulfide of sodium 0. 10

Glycerine 150 C 9. 6 Glycerine +2 pereentum liver of sulfur 0. 02

Glycerine:wster=l:l 100 C l4. 2 Glycerine:water=l:l 100 C+0.2 percentum sulfide of sodium.. 0.03

Glycol:water=l:l 100 C 6. 2 Glycolzwater=lzl 100 C.+0.l percentum liver of sullun. 0. 05 0 (4) A permanent improvement in the resistmce to corrosion of articles of magnesium and iigh grade magnesium alloys may be produced recording to the present invention by the fornation of a protective layer containing magiesium sulfide upon the surface of the articles iy means of a treatment with an aqueous solu- ;ion of alkali sulfides.

Sheets of a magnesium alloy are immersed in I. 0.5 to 1 percent aqueous solution of sulfide of :odium at temperatures between and 100 C. luring a few hours. This treatment results in ;he formation of a protective layer containing nagnesium sulfide and having a color ranging Jetween brown and black. I1 sheets treated in ;his manner are subsequently exposed to corrodng agents, for example to the action of ordinary water, a marked improvement is noticed when :ompared with sheets which have not -been treat- :d in this manner. The following table gives a iumerical comparison of the behavior of two sheets made from the same alloy, one sheet havng remained untreated whereas the other was provided with a protective surface layer by imnersing it in a 0.5 percent aqueous solution of sodium sulfide at C. for 10 hours. The sheets were then brought into contact with ordinary water and the amount of hydrogen developedrepresenting the progress of corrosionascerbained after 24, 48 and 72 hours respectively.

Development of H1 in liters per sq. meter surface After hours Treated sheets Untreated sheets I claim:-

1. A non-corrosive liquid for use in conjunction with articles consisting of magnesium and high grade magnesium alloys, characterized by the presence of a major quantity of a polyhydric alcohol and a minor quantity ranging between about 0.1 and 3 percent of an alkali sulfide in said liquid.

2. The method of protecting magnesium and magnesium alloys from the action of an alcoholcontaining liquid in contact therewith, by adding to said alcohol-containing liquid alkali sulphide in amount suflicient to substantially prevent corrosion of the metal. i

3. The method of counteracting the corroding influence of a polyhydric alcohol on magnesium and magnesium alloys with which said alcohol is in contact, by adding to said alcohol an amount of alkali sulphide suflicient to prevent substantial corrosion of the metal.

4. The method of counteracting the corroding influence of liquid medium upon magnesium and magnesium alloys which are in contact there with, by adding to said medium alkali sulphide in amount sufficient to prevent substantial corrosion of the metal.

J OSEF MARTIN MICHEL. 

